Output driver equipped with a sensing resistor for measuring the current in the output driver

ABSTRACT

An electronic circuit has an output driver (DRV) for providing a driving signal (U 0 ). The output driver has a transistor (T) with a first main terminal, a second main terminal and a control terminal coupled to receive a control signal (V cntrl ), a power supply terminal (V SS ), an output terminal (OUT) for providing the driving signal (U 0 ) that is coupled to the second main terminal, and a sensing resistor (R m ) coupled between the power supply terminal (V SS ) and the first main terminal. The output driver (DRV) further has means for temporarily disabling the coupling between the control terminal and the control signal (V cntrl ) during a peak voltage across the sensing resistor (R m ). The means may have a circuit that has a unidirectional current behavior, such as a diode (D), in series with the control terminal of the transistor (T).

The invention relates to an electronic circuit comprising an outputdriver for providing a driving signal, which output driver comprises atransistor with a first main electrode, a second main electrode and acontrol electrode coupled to receive a control signal, a supplyterminal, an output terminal to provide the driving signal, which outputterminal is coupled to the second electrode, and a sensing resistorwhich is coupled between the supply terminal and the first mainelectrode.

Such an electronic circuit is known from the general state of the art,as shown in FIG. 1 and FIG. 2. FIG. 1 shows an output driver DRV. Saidoutput driver DRV receives a supply voltage between a supply terminalV_(SS) and a further supply terminal V_(DD), and provides an outputvoltage U₀ to an output terminal OUT. The output driver DRV comprises anoutput transistor T and a further output transistor T_(F), which arecontrolled by, respectively, a control circuit CNTRL and a furthercontrol circuit CNTRL_(F). The output transistor T and the furtheroutput transistor T_(F) are arranged in series between the supplyterminal V_(SS) and the further supply terminal V_(DD). The controlcircuit CNTRL supplies a binary control signal V_(CNTRL). The furthercontrol circuit CNTRL_(F) similarly supplies a control signal to thegate of the further transistor T_(F) which is in phase with the controlsignal V_(CNTRL). As a result, the transistors T and T_(F) arealternately conducting or non-conducting. For certain applications it isdesirable to know the current through the output driver DRV. For thispurpose, in the general state of the art as shown in FIG. 1, a sensingresistor R_(M) is arranged in series with the source of the field effecttransistor T. If the field effect transistor T carries a current, avoltage is applied across the sensing resistor R_(M), which is a measureof the current through the field effect transistor T, which current canbe calculated using Ohm's law. The voltage across the resistor R_(M) hasthe adverse effect, however, that it causes the gate-source voltage ofthe field effect transistor T to be reduced. As a result, the maximumdriving power of the driver DRV is reduced. In accordance with thegeneral state of the art, this problem is frequently solved by thecircuit as shown in FIG. 2. This means that the sensing resistor R_(M)is arranged in series only with the supply terminal V_(SS), while thefull control voltage V_(CNTRL) is available between the gate and thesource of the field effect transistor T. On the assumption that thefurther supply terminal V_(DD) receives a positive voltage with respectto the supply terminal V_(SS), the voltage across the resistor R_(M) isnegative with respect to ground, or with respect to the substrate if anintegrated circuit is used. This is undesirable, in particular, when theresistor is used in an integrated circuit because signal processing ofthe measured voltage becomes much more complicated.

Therefore, it is an object of the invention to provide an electroniccircuit with a driver, which is provided with a sensing resistor formeasuring the current through the driver, wherein the available controlvoltage of the output transistor is reduced hardly, if at all, by thevoltage across the sensing resistor, and wherein signal processing ofthe measured voltage is possible in a simple manner.

To achieve this object, the output driver further comprises means forrendering ineffective said coupling between the control electrode andthe control signal during a peak voltage across the sensing resistor.Most of the time substantially no current flows through the transistor.Only for a comparatively short period of time, i.e. when the drivingsignal changes from a high logic value to a low logic value, a currentof significant value flows through the transistor, thereby causing apeak voltage across the sensing resistor. During this peak voltage, thecontrol electrode, or the gate if a field effect transistor is used, isdecoupled from the control circuit that controls this control electrodemost of the time. The ever-present intrinsic capacitance between gateand source of the transistor makes sure that the voltage differencebetween gate and source of the transistor remains substantially constantduring this peak voltage across the sensing resistor. The potential onthe gate of the transistor consequently exhibits a peak that correspondsto the peak voltage across the sensing resistor.

In an embodiment of an electronic circuit comprising an output driver inaccordance with the invention said means comprises a unipolar circuitthat is arranged in series with the control electrode of the transistor.This unipolar circuit makes sure that the coupling between the gate ofthe transistor and the control circuit is automatically interrupted assoon as the potential on the gate of the transistor approximates themaximum potential that can be supplied by the control circuit as aresult of the peak voltage across the sensing resistor which alsoaffects the gate of the transistor via its gate-source capacitance.

In an embodiment of an electronic circuit comprising an output driver inaccordance with the invention the unipolar circuit comprises a diode.Said diode is a very simple implementation for the unipolar circuit.

In an embodiment of an electronic circuit comprising an output driver inaccordance with the invention the output driver further comprisescurrent means which are coupled between a further supply terminal andthe control electrode of the transistor. The current means can be usedto compensate for the possible slow leaking away of the gate-sourcevoltage of the transistor during the time that the gate of thetransistor is decoupled from the control circuit. The value of thecurrent to be supplied by the current means is very small. Consequently,a significant increase in power dissipation of the output driver doesnot take place. As the value for this small current may easily be chosento be slightly too high, the potential on the gate of the transistor canbecome impermissibly high. This can be precluded by providing the outputdriver with limiting means for limiting the potential on the controlelectrode of the transistor. Said limiting means can be equipped, forexample, with a zener diode that is coupled between the gate of thetransistor and the supply terminal, or between the gate and the sourceof the transistor.

These and other aspects of the invention are apparent from and will beelucidated with reference to the embodiments described hereinafter.

In the Drawings:

FIG. 1 shows a known electronic circuit comprising an output driverequipped with a sensing resistor for measuring the current through theoutput driver;

FIG. 2 shows an alternative, known electronic circuit comprising anoutput driver equipped with a sensing resistor for measuring the currentthrough the output driver;

FIG. 3 shows a first embodiment of an electronic circuit comprising anoutput driver equipped with a sensing resistor for measuring the currentthrough the output driver in accordance with the invention;

FIG. 4 shows a second embodiment of an electronic circuit comprising anoutput driver equipped with a sensing resistor for measuring the currentthrough the output driver in accordance with the invention; and

FIG. 5 shows a third embodiment of an electronic circuit comprising anoutput driver equipped with a sensing resistor for measuring the currentthrough the output driver in accordance with the invention.

In these Figures, like reference numerals refer to like parts orelements.

FIG. 3 shows a first embodiment of an electronic circuit including anoutput driver DRV comprising a supply terminal V_(SS), a further supplyterminal V_(DD), an output field effect transistor T, a further outputfield effect transistor T_(F), a control circuit CNTRL, a furthercontrol circuit CNTRL_(F), a diode D, a sensing resistor R_(M), and anoutput terminal OUT for supplying a driving signal U₀. The controlcircuit CNTRL supplies a control signal V_(CNTRL) which can be suppliedvia the diode D to the gate of the transistor T. The further controlcircuit CNTRL_(F) directly supplies a control signal to the furthertransistor T_(F). The sensing resistor R_(M) is coupled between thesource of the transistor T and the supply terminal V_(SS) that is alsoconnected to ground. Like the drain of transistor T_(F), the drain oftransistor T is coupled to the output terminal OUT. The source of thefurther transistor T_(F) is connected to the further supply terminalV_(DD). Next, a description will be given of the operation of thecircuit. On the assumption that no significant DC load is coupled to theoutput terminal OUT, the transistors T and T_(F) will not carry currentif the driving signal U₀ has a high logic value or a low logic value fora good length of time. The transistor T only carries current in thetransition phase from the high to the low logic value of the drivingsignal U₀. As a result, the control signal V_(CNTRL) would diminish inthe known circuit in accordance with FIG. 1. This does not occur howeverin the circuit shown in FIG. 3. This can be attributed to the fact thatat that instant the diode D is turned off. At said instant, the voltageV_(P) between the gate of the transistor T and the supply terminalV_(SS) is decoupled from the control voltage V_(CNTRL). As a result, thepotential on the gate will follow the potential on the source of thetransistor T, so that the gate-source voltage of the transistor Tremains substantially constant. This has the advantage that the maximumoutput current that can be drawn from the output terminal OUT in thetransistor T is not limited.

FIG. 4 shows a second embodiment of an electronic circuit comprising anoutput driver DRV. The control circuit CNTRL is equipped with aconventional inverter comprising an N-type field effect transistor and aP-type field effect transistor, with the diode D of FIG. 3 beingarranged, in this case, in series with the drain of said P-type fieldeffect transistor. The terminal 1 receives the input signal, andterminal 2 receives a supply voltage. The operation of the circuit issimilar to that of the circuit shown in FIG. 3. Most of the time, i.e.when there is no peak voltage across the sensing resistor R_(M), thevoltage between the gate and the source is determined by V_(CNTRL),which is fully imposed by the control circuit CNTRL. During a peakvoltage across the sensing resistor R_(M), the diode D is reversebiased, as a result of which the signal between the gate and the sourceis not dictated by the control circuit CNTRL but by the transistor Titself. In this period, the voltage V_(P) thus corresponds to thevoltage V_(P) as indicated in FIG. 3.

FIG. 5 shows a third embodiment of an electronic circuit comprising anoutput driver DRV. During the peak voltage across the sensing resistorR_(M), the gate-source voltage of the field effect transistor T remainsrelatively constant as a result of the gate-source capacitance. However,the voltage decreases gradually as a result of leakage. This can becompensated for by using current means which, in FIG. 5, are implementedusing the current source J that is coupled to the gate of the fieldeffect transistor T. The current value supplied by the current source Jmust be very small. If the current value is too large, the potential onthe gate of the field effect transistor T may become impermissibly high.To preclude this, a zener diode may be coupled between the gate and thesupply terminal V_(SS), or a zener diode may be coupled between the gateand the source of the field effect transistor T. The latter solution ispreferred as it leads to a direct limitation of the gate-source voltage.

The electronic circuit in accordance with the invention can be appliedin an integrated circuit or implemented in a discrete manner. Althoughfield effect transistors are applied in all embodiments, it isalternatively possible to substitute, for example, the output transistorT with a bipolar transistor. In that case, the current value to besupplied by the current source J must be much higher in general. It isalso possible to substitute all P-type transistors with N-typetransistors, provided also all N-type transistors are replaced by P-typetransistors, the diode's polarity is reversed, and the supply voltagepolarity is reversed.

1. An electronic circuit having an output driver for providing a drivingsignal, the output driver comprising: a supply terminal; a controlcircuit coupled to the supply terminal and configured and arranged toapply a voltage to a transistor control electrode; at least twotransistors, each transistor having a first main electrode, a secondmain electrode and a control electrode, the second main electrode ofeach of the at least two transistors being coupled together, and thecontrol electrode of at least one transistor being coupled to receive acontrol signal from the control circuit, and the first main electrode ofsaid at least one transistor being coupled to the supply terminal; asensing resistor coupled between the supply terminal and the first mainelectrode of said at least one transistor; and an output terminalcoupled to the second main electrode to provide the driving signal, anda unipolar circuit between the control circuit and the control electrodeof said at least one transistor to decouple the control electrode fromthe control circuit during a peak voltage condition across the sensingresistor, wherein the supply terminal is coupled to the control circuitduring the peak voltage condition wherein the control circuit includesan inverter having an N-type field effect transistor and a P-type fieldeffect transistor, the unipolar circuit arranged in series with a drainof the P-type field effect transistor.
 2. An electronic circuit asclaimed in claim 1, further comprising a further supply terminal and acurrent source coupled between the control electrode of the at least onetransistor and the further supply terminal, wherein the current sourcesupplies a current to the control electrode of the at least onetransistor.
 3. An electronic circuit as claimed in claim 2, furthercomprising a zener diode coupled between the control electrode of the atleast one transistor and the supply terminal, wherein the zener diodelimits a voltage between the control electrode of the at least onetransistor and the first main electrode of the at least one transistor.4. An electronic circuit having an output driver for providing a drivingsignal, the output driver comprising: at least two transistors, eachtransistor having a first main electrode, a second main electrode and acontrol electrode, the second main electrode of each of the at least twotransistors being coupled together, and the control electrode of atleast one transistor being coupled to receive a control signal; a supplyterminal; an output terminal to provide the driving signal, the outputterminal being coupled to the second main electrode; a sensing resistorwhich is coupled between the supply terminal and the first mainelectrode of one transistor of the at least two transistors; means forrendering ineffective said coupling between the control electrode andthe control signal during a peak voltage across the sensing resistor,said means being a unipolar circuit arranged in series with the controlelectrode of the one transistor; and a control circuit that supplies thecontrol signal, the control circuit including an inverter having anN-type field effect transistor and a P-type field effect transistor, theunipolar circuit being arranged in series with a drain of the P-typefield effect transistor.